Antennas mounted on a moving article, such as a television broadcast receiving antenna mounted on a car, may be non-directional. Non-directional antennas include, for example, an Alford loop antenna and a cloverleaf antenna. To receive radio waves in, for example, VHF and UHF bands by means of such non-directional antennas, one for each of the frequency bands has been used.
An Alford loop antenna and a cloverleaf antenna are formed of many components, are large in size and require complicated manufacturing processes. Accordingly, such antennas for receiving UHF and VHF bands undesirably require a large space to mount them because they are large. In addition, non-directional antennas, such as Alford loop antennas and cloverleaf antennas, are subject to receiving undesired radio waves and, therefore, tend to cause ghosts to appear in a television picture when used for receiving television broadcast ratio waves.
An object of the present invention is to provide an antenna which is small in size and can selectively receive radio waves of plural frequency bands. Another object is to provide an antenna which hardly receives undesired radio waves and substantially non-directional in receiving radio waves.
An antenna system according to one embodiment of the present invention includes a dipole antenna for a first frequency band. The dipole antenna has a pair of rod elements arranged substantially in a straight line. The antenna system also includes a Yagi antenna for a second frequency band higher than the first frequency band, which has a radiator disposed on at least one of the pair of rod elements of the dipole antenna. The first and second frequency bands may be the VHF and UHF bands, respectively.
The Yagi antenna may include, in addition to the radiator, a director and/or a reflector. The Yagi antenna radiator may be disposed at a predetermined angle, e.g. 90xc2x0, with respect to the rod elements of the dipole antenna. The radiator may be a folded-dipole antenna. It is desirable to dispose the folded-dipole antenna in such a manner that its longitudinal center is on the rod element of the dipole antenna. The radiator of the Yagi antenna may be a planar radiator.
A plurality of such dipole antennas may be used for the first frequency band. In this case, the rod elements of the different dipole antennas are disposed to extend radially from the same center, and a plurality of Yagi antennas are used with their radiators disposed on the rod elements of the dipole antennas. The radiators may be disposed on the respective rod elements of at least one dipole antenna, or may be disposed on different ones of the dipole antennas. Selecting means selects one of the outputs of the dipole antennas and also one of the outputs of the Yagi antennas. The selecting means may be arranged to select more than one outputs of the antennas.
According to another embodiment of the present invention, an antenna system includes a first Yagi antenna for a first frequency band having at least one director, and a plurality of second Yagi antennas for a second frequency band higher than the first frequency band. The second Yagi antennas have radiators disposed on the at least one director of the first Yagi antenna. The first Yagi antenna may have a plurality of directors. The radiators of the second Yagi antennas may be disposed on one director or on different ones of the directors. The first Yagi antenna also has a radiator. It may have a reflector, too.
An antenna system according to a further embodiment of the present invention includes a first antenna for a first frequency band including a pair of rod elements mounted on a boom, and a second antenna for a second frequency band higher than the first frequency band. The second antenna has a radiator which is mounted on the boom substantially in parallel with the rod elements of the first antenna and can be used as a director of the first antenna.
An antenna system according to a still further embodiment includes a body and a plurality of Yagi antennas all for the same frequency band disposed in the body. The Yagi antennas are arranged at different levels in the body. The Yagi antennas are disposed in the body to be receptive of radio waves from different directions. Parts of the respective antennas intersect without contacting each other. Any one of various shapes may be employed for the body, but a planar body is preferred for space saving. Each Yagi antenna may include at least a radiator and a reflector or at least a radiator and a director. The intersecting parts of the respective Yagi antennas may be parts of the radiators, or the reflectors for the antennas with reflectors.
A plurality of Yagi antennas may be disposed in the plane at each of the different levels for receiving radio waves from different directions.
The plurality of Yagi antennas in one plane may be two Yagi antennas arranged to receive radio waves from opposite directions. In this case, each antenna has a radiator and a reflector. Each radiator is in a flaring shape with its opposite extremities located close to corresponding extremities of the other radiator. Each reflector includes a portion curving toward the associated radiator.
According to a still further embodiment, an antenna system includes a body and a plurality of Yagi antennas disposed in said body for receiving radio waves from different directions. Each of the Yagi antennas has to have only a radiator and a reflector or a radiator and a director. Adjacent ones of the Yagi antennas are at different levels, and Yagi antennas adapted to receive radio waves from directions 180xc2x0 apart from each other are disposed on the same plane.
Each of the Yagi antennas may be connected to a matching device. Selecting means selects one of outputs of the matching devices of the respective Yagi antennas and combinations of outputs of the matching devices of pairs of adjacent Yagi antennas. Each of the matching devices has first and second output terminals with the first output terminal connected to a reference potential and with the second output terminal connected to the selecting means. With this arrangement, the phases of the antenna outputs provided from the respective matching devices are aligned.
According to a further embodiment of the present invention, an antenna system includes an even number of rod antennas radially extending substantially in the same plane. The number of the rod antennas is equal to or greater than four. The antenna system further includes pairs of feed terminals as many as the rod antennas. Each pair of feed terminals are led from a pair of adjacent ones of the rod antennas, whereby V-shaped antennas as many as the rod antennas are provided.
A matching device may be connected to each pair of the feed terminals. Selecting means selects individual ones of outputs from the respective matching devices and combinations of outputs of the matching devices of pairs of adjacent V-shaped antennas. Also, energizing means is provided for energizing only the selected one or more of the matching devices.
In this embodiment, each rod antenna functions as a component of two V-shaped antennas. Accordingly, in order to avoid its adverse effect, the matching devices associated with the respective antennas operate only when the associated antennas are selected by the selecting means.
Each matching device has first and second output terminals with the first output terminal connected to a reference potential and with the second output terminal connected to the selecting means.
An antenna system according to a further embodiment of the present invention includes a plurality of radially extending rod antennas. Pairs of rod antennas extending diametrically opposite directions substantially in the same plane form a plurality of dipole antennas. The antenna system further includes a pair of feed terminals led from each dipole antenna.
A matching device may be connected to each pair of the feed terminals. Selecting means selects individual ones of outputs from the respective matching devices and combinations of outputs of the matching devices of pairs of adjacent dipole antennas. Also, energizing means is provided for energizing only the selected one or ones of the matching devices.
In this embodiment, too, each rod antenna functions as a component of two dipole antennas. Accordingly, in order to avoid its adverse effect, the matching devices associated with the respective antennas operate only when the associated antennas are selected by the selecting means.
Each matching device has first and second output terminals with the first output terminal connected to a reference potential and with the second output terminal connected to the selecting means.
In accordance with a still another embodiment of the present invention, an antenna system includes a plurality of antennas for the same frequency band disposed to be receptive of radio waves from different directions, selecting means for selecting individual ones of outputs from the respective antennas and combinations of outputs of pairs of adjacent antennas, and control means for providing a selection control signal to the selecting means.
According to a further embodiment of the present invention, an antenna system includes a plurality of first antennas for a first frequency band disposed to be receptive of radio waves in the first frequency band coming from different directions, and a plurality of second antennas for a second frequency band disposed to be receptive of radio waves in the second frequency band coming from different directions and associated with the respective ones of said first antennas. The antenna system further includes the same number of amplifying means as the first and second antennas, each of which receives outputs of one of the first antennas and an associated one of the second antennas. Combining means combines outputs of the amplifying means. The antenna system further includes control means for switching between a state in which the amplifying means are individually energized and a state in which the amplifying means receiving the outputs of pairs of adjacent ones of the first antennas are energized.
According to a still further embodiment of the present invention, a plurality of Yagi antennas are so disposed at different levels in a body as to be receptive of radio waves in a first frequency band coming from various directions. A plurality of rod antennas are disposed at a level different from the levels of the Yagi antennas. The rod antennas are disposed to be receptive of radio waves in a second frequency band coming from various directions. A plurality of Yagi antennas may be disposed at the same level for receiving radio waves from different directions. Also, an even number, not smaller than four, of rod antennas may be used and combined to form V-shaped antennas or dipole antennas. The level at which the rod antennas are disposed may be intermediate between the levels at which the Yagi antennas are disposed.